In Opposing Force, the player returns to Black Mesa as Corporal Adrian Shephard, one of the soldiers sent in to eliminate the civilians from the original game. On an undisclosed mission to Black Mesa, things don't quite go as planned, and Shephard gets separated from his unit at the beginning of the game.

Now alone and without any orders, Shephard must face not only a new alien threat, but also the government agents sent in after the soldiers pull out. Their mission is to cover up the incident by destroying the facility and eliminating everything on their way, including the soldiers who were left behind.

In the single-player campaign, some of these weapons replace their Half-Life counterparts (Desert Eagle - .357 Magnum, M40A1 - Crossbow, Pipe wrench/Combat knife - Crowbar, Shock Roach - Hivehand). However, in the multiplayer game the player can carry both the new Opposing Force weapons and their Half-Life variants at the same time, while cheat codes also make it possible to possess and use these weapons in single-player mode.

Half-Life: Opposing Force was announced on April 15, 1999 to be developed by Gearbox Software.[2] At the time, Gearbox was a new company formed by the core members of the defunct Rebel Boat Rocker which had been shut down after a struggle with their publisher on their first video game project. The team wanted to work on small scale titles based on an existing technology rather than creating a completely new game.[3]

Meanwhile, Valve was busy with developing Half-Life 2 and its supporting engine technology.[4] They were looking for someone to take over their role and expand the Half-Life universe so they could focus on their future titles. Gearbox had connections at Valve and were interested in meeting them to see what they could put together. However, Gabe Newell ended up calling them first as he a similar idea for a project. Through their connections, Gabe was told what Gearbox was doing, and he believed they'd be a good fit. Both companies got together to discuss and explore the possibilities.[5]

Randy Pitchford pitched the idea of player returning to Black Mesa as one of the soldiers invading the facility occurring during the same time as the events of Half-Life, retelling the story from another character's perspective. He likened the concept to the play Rosencrantz and Guildenstern Are Dead.[4] Newell, inspired by The Alexandria Quartet series of novels, also wanted to have a Rashomon-esque structure and allow the players to experience the same events from a different point of view.[6] The deal was signed, and the production of the Opposing Force expansion pack went forward.

Opposing Force received a very favorable reception from critics, holding a score of 85.45% on the review aggregator site GameRankings.[7] Although figures for the game's sales on Steam have not been released, Opposing Force has sold over 1.1 million copies at retail.[8]

Computer and Video Games reviewer Kim Randell noted that "Gearbox has obviously gone to great pains to provide a similar experience to the original". Praise was also given to the game's multiplayer; Randell stated that the new additions for multiplayer made it the area of Opposing Force that "really shines". Randall closed the review by concluding that Opposing Force is "an awesome achievement".[9]Erik Wolpaw, writing for GameSpot, noted that as most expansion packs were mediocre, "it's appropriate that Gearbox Software's Opposing Force, the official expansion for the genre-redefining Half-Life, in turn sets a new standard of quality for future action-game mission packs". Wolpaw praised the design of the single-player campaign, commenting that "you can sense the designers' enthusiasm as one memorable scene unfolds after another, and it compels you to keep playing". Although criticizing some elements of the game's artificial intelligence and describing some of the new models as "merely window dressing", the review concluded that Opposing Force was an "impassioned application of creative design".[10]

Other reviews echoed many of the positive aspects of the game. GamePro stated that "Gearbox has done one hell of a job in creating not just an add-on for Half-Life, but a continuation of a masterpiece", praising both level design and story elements, but noted that it was a little too short.[11] However, some critics dissented on the idea that Opposing Force was as influential as other reviewers made out.

PC Zone stated that "the taste left in the mouth is a bitter one", noting that "Opposing Force is a few excellent ideas strung together by pedestrian Half-Life padding", but concluded that "it was still a good weekend's worth of entertainment".[12] Eurogamer stated that Opposing Force still had similar problems to other expansion packs, commenting that "χ amount of new content has been created and it is going to be cut into the old content in a linear way to make it look like an all new game", but noted that "fortunately though the new stuff in Opposing Force... is pretty damn good". Although praising the level design as the game's strongest point, the reviewer felt that "towards the end of the game... they were running out of development time".[13] Reviewing for IGN, Vincent Lopez stated that the game "does a fantastic job of making you remember exactly why you enjoyed the original so much", but criticized this as the biggest drawback, commenting that "you may find yourself wishing for a more original experience", but concluded that "for good, and bad: it's good to be back".[14] The game won several publication awards,[11] as well as the Computer Action Game of the Year Interactive Achievement Award of 2000 from the Academy of Interactive Arts & Sciences.[15]

If the player follows Gordon Freeman through the portal to Xen in We Are Not Alone, the game will end, accusing the player of trying to rewrite the history with a game over message that says: "Evaluation terminated, Subject attempted to create a temporal paradox". This means what happened in the original level was changed by the player.